1. Field of the Invention
The present invention relates to a sinker electric discharge machining device for forming a required cavity, recess or hole, etc. in a workpiece by intermittently causing electrical discharge in a machining gap formed between a tool electrode and a conductive workpiece. In particular, the present invention relates to a power supply device for sinker electric discharge machining, for supplying a sequence of controlled current pulses to the machining gap.
2. Description of the Related Art
Generally, with a sinker electric discharge machining apparatus, a metallic workpiece is located in a tank, and a machining gap is filled with dielectric fluid. If a power supply applies a voltage to the machining gap, the dielectric fluid is ionized. Then, electric discharge is generated across the machining gap and an ON time (alternatively called duration time) starts. During the ON time, discharge current flows through the machining gap, and a small portion of the workpiece material is removed. Once the ON time is completed, the power supply stops application of the current pulse and an OFF time (also called pause time) starts. During the OFF time insulation of the machining gap is restored. Once the OFF time is completed, the power supply again applies a voltage to the machining gap. In this manner electric discharge is repeatedly generated across the machining gap. Sinker electric discharge machining is a process for accurately forming a cavity in a workpiece using electrical discharge while continuously advancing a tool electrode towards the workpiece.
Current pulses having larger energy remove more material from the workpiece. The energy of a current pulse is mainly dependent on the ON time and peak current of the current pulse. Larger energy current pulses improve the material removal rate. On the other hand, smaller energy current pulses are used to improve dimensional precision and surface roughness of a cavity.
Generally, a sinker electric discharge machining process is divided into a plurality of steps of differing current pulse energy. In a first step, namely rough machining, current pulses having large energy are supplied to the machining gap. As the process proceeds to subsequent steps the energy of the current pulses is reduced in a stepwise fashion. In this way dimensional precision of a cavity is improved to a dozen of μm or better. In a final step, or finish machining, surface roughness of the cavity is improved to a dozen of μmRz or better.
A current pulse waveform has an impact on wear of the tool electrode. An ideal waveform for the current pulses differs depending on tool electrode material, workpiece material and the size of a cavity. In the case of machining an iron (Fe) based alloy workpiece using a copper (Cu) tool electrode, current pulses having a long ON time compared to the peak current reduce wear rate. When using such specialized current pulses, the tool electrode is connected to a positive electrode of a dc power supply. Wear rate is usually a rate of wear of the tool electrode (g) with respect to removal of the workpiece (g). Electric discharge machining carried out with a wear rate of 0.1% or less is known as “no wear”. Current pulses having a gradual rise time are also known to lower wear rate.
In the case where the specialized current pulses having a long ON time are repeatedly supplied to the machining gap, a sufficiently long OFF time is required in order to restore the insulation of the machining gap. However, if the OFF time becomes longer, the material removal rate is lowered. The desired waveform of the current pulses will differ depending on whether priority is placed on a low wear rate or high material removal rate.
Ordinarily, in cases where a cemented carbide alloy workpiece is machined using a copper tungsten (CuW) tool electrode, a high material removal rate is important, and current pulses having a high peak current are applied to the machining gap at high frequency. A typical cemented carbide is a material having tungsten carbide (WC) powder sintered together with a binding agent of cobalt (Co). In a rough machining step, current pulses having a high peak current of 20 A or more and a short ON time of about 10 μm are supplied to the machining gap. In order to increase the frequency of repeating electric discharge, it is desirable for the rising edge and falling edge of the current pulses to be steep.
U.S. Pat. No. 5,126,525 discloses a power supply device for sinker electric discharge machining in which a current limiting resistor is excluded from an electric discharge machining circuit. Current flowing through a machining gap (“gap current”) is interrupted at regular intervals after reaching a set peak current. The gap current is kept substantially constant by this chopping. This type of power supply device can supply current pulses having a steep rising edge, and also does not suffer energy loss due to current limiting resistor. The steep rising edge of the current pulses improves the frequency of the current pulses.